Nearly all reverse osmosis plants operated for the desalination of sea water in order to produce drinking water in industrial scale are equipped with an energy recovery system based on turbines. These are activated by the concentrate (brine) leaving the plant and transfer the energy contained in the high pressure of this concentrate usually mechanically to the high-pressure pump. In the pressure exchange system PES the energy contained in the brine is transferred hydraulically and with an efficiency of approximately 98% to the feed. This reduces the energy demand for the desalination process significantly and thus the operating costs. At the same time this advanced technological approach allows for reducing the size of the high-pressure pump to an amount of feed water equivalent to the amount of permeate produced, saving investment costs. The installation of PES in existing installations with a permeate production of more than 2,000m(3)/d allows for short amortisation times. The concept of the pressure exchange system PES and the operating data of the demonstration plant installed in the reverse osmosis unit INALSA I on Lanzarote, Spain with a permeate production of 5,000 m(3)/d are presented.